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Course A Basics of Water Resources Pieter van der Zaag, UNESCO-IHE Delft & University of Zimbabwe Table of Contents 1.3 Integrated water resources management 6 2.6 The water balance as

Basics of Water Resources

Course book

Course A

CATALICAdvice and Management in International Co operation

WaterNet, in collaboration with the Centre of Conflict Resolution CCR (South Africa), the Instituto Superior

de Relações Internacionais ISRI (Higher Institute of International Relations) (Mozambique), Catalic (The Netherlands/Mozambique), UNESCO-IHE Delft (The Netherlands) and the University of Zimbabwe

(Zimbabwe), has developed

a 3 day course on

Basics of Water Resources

The aim of the course is to introduce the basics of water resources to non-water managers, in order for them to

be able to communicate more meaningfully with water engineers, hydrologists etc

The specific objectives of the course are:

a to introduce the basics of water resources

b to improve communication between non-water professionals and water professionals

The subjects addressed include:

- Concepts and definitions

- Water resources

- Water allocation principles

- Urban water demand

- Agricultural water demand

- Environmental water requirements

The course is targeting non-water professionals and stakeholder representatives

The course has been developed under the UNESCO and Green Cross programme "From Potential Conflict to Cooperation Potential: Water For Peace", which forms part of the World Water Assessment Programme WWAP

The course materials consist of a course book

Course A

Basics of Water Resources

Pieter van der Zaag, UNESCO-IHE Delft & University of Zimbabwe

Table of Contents

1.3 Integrated water resources management 6

2.6 The water balance as a result of human interference 31

5 Agricultural water demand 70

5.3 Yield reduction due to water shortage 79

1 Concepts and definitions

1.1 The water cycle

Water is finite on earth There is a fixed amount of water which neither decreases or increases Fresh water is a renewable resource because of the water cycle From a human perspective the source of freshwater is rainfall Most of this rainfall is used directly for vegetative growth, such as natural vegetation, pasture, rain-fed maize etc This process, known as transpiration, is highly productive and produces in Southern Africa the bulk of food crops

Only a small portion of the rainfall flows into rivers as surface water and recharges groundwater (Figure 1.2) This water is used for domestic water supply, industrial production, irrigated agriculture etc This is the water that we tend to harness through infrastructure development (e.g dams, wells) and that we tend to pollute

If we talk about Integrated Water Resources Management, we mean to consider the entire water cycle This means that we also look at rain-fed agriculture production, soil and water conservation within the watershed, rainwater harvesting techniques etc

To facilitate the comprehensive thinking in terms of the entire water cycle, three types of water can be distinguished, together forming the 'rainbow' of water

Figure 1.2 Schematic water balance for Southern Africa, showing the average

A rainbow of water

The rainbow of water distinguishes three types of water depending on their occurrence in

the water cycle (Figure 1.3)

• ‘white’ water = rainfall and that part of rainfall which is intercepted and immediately evaporates back to the atmosphere

• ‘blue’ water = water involved in the runoff (sub-)cycle, consisting of surface water and groundwater (below the unsaturated zone)

• ‘green’ water = water stemming directly from rainfall, that is transpired by vegetation (after having been stored in the unsaturated zone) (Falkenmark, 1995)

surface runoff

groundwater

runoff

“blue water”

seepage percolation

Water use

There are a large number of types of

water use Among these are:

• Industrial and commercial use

• Institutions (e.g schools, hospitals,

government buildings, sports

facilities etc.)

• Waste and wastewater disposal

• Cooling (e.g for thermal power

Demand for, and use of water

Demand for water is the amount of water required at a certain point The use of water

refers to the actual amount reached at that point

We can distinguish withdrawal uses and non-withdrawal (such as navigation, recreation, waste water disposal by dilution) uses; as well as consumptive and non-consumptive uses

Consumptive use is the portion of the water withdrawn that is no longer available for

further use because of evaporation, transpiration, incorporation in manufactured products and crops, use by human beings and livestock, or pollution

The terms “consumption”, “use” and “demand” are often confused The amount of water actually reaching the point where it is required will often differ from the amount required Only a portion of the water used is actually consumed, i.e lost from the water resource system

A similar confusion exists when talking about water losses It depends on the scale

whether water is considered a loss or not At the global scale, no water is ever lost At the scale of an irrigation scheme, a water distribution efficiency of 60% indeed means that slightly less than half of the water is lost Part of this water, however, may return to the river and be available to a downstream user At the scale of the catchment, therefore, it is the transpiration of crops (60% in this example) that can be considered a loss!

While the total available freshwater is limited (finite), demand grows Hence the

importance of water resources management

The value of water

The various uses of water in the different sectors of an economy add value to these sectors

Some sectors may use little water but contribute significantly to the gross national product

(GNP) of an economy (see Table) Other sectors may use a lot of water but contribute

relatively little to that economy The added value of some uses of water are difficult, if not

impossible to measure Consider for instance the domestic use of water: how to quantify

the value of an adequate water supply to this sector?

Table 1.1 The contribution of various sectors in the economy of Namibia to Gross

The damage to an economy by water shortage may be immense It is well known, for

instance, that a positive correlation exists between the Zimbabwe stock exchange index

and rainfall in Zimbabwe The drought of 1991/92 had a huge negative impact on the

Zimbabwean economy (see box 1.1)

Box 1.1: The impact of drought in Zimbabwe

During the drought of 1991/92, the country’s agriculture production fell by 40 % and 50%

of its population had to be given relief food and emergency water supplies, through

massive deep drilling programmes, since many rural boreholes and wells dried up Urban

water supplies were severely limited with unprecedented rationing Electricity generation

at Kariba fell by 15% causing severe load shedding As a result its GDP fell by 11%

1.2 Three characteristics of water

Water has at least three important physical attributes with a bearing on management:

• Fresh water is vital to sustain life, for which there is no substitute This means that water has a (high) value to its users

• Although water is a renewable resource, it is practically speaking finite The use of water is therefore subtractible, meaning that the use by somebody may preclude the use

by somebody else

• Water is a fugitive resource It is therefore difficult to assess the (variations in) stock and flow of the resource, and to define the boundaries of the resource, which complicate the planning and monitoring of withdrawals as well as the exclusion of non-members The vital nature of water gives it characteristics of a public good

Its finite nature confers to it properties of a private good, as it can be privately

appropriated and enjoyed

The fugitive nature of water, and the resulting high costs of exclusion, confers to it

properties of a common pool resource

Water resources management aims to reconcile these various attributes of water This is

obviously not a simple task The property regime and management arrangements of a

water resources system are therefore often complex

1.3 Integrated water resources management

There is growing awareness that comprehensive water resources management is needed, because:

• fresh water resources are limited;

• those limited fresh water resources are becoming more and more polluted, rendering them unfit for human consumption and also unfit to sustain the ecosystem;

• those limited fresh water resources have to be divided amongst the competing needs and demands in a society

• many citizens do not as yet have access to sufficient and safe fresh water resources

• techniques used to control water (such as dams and dikes) may often have undesirable consequences on the environment

• there is an intimate relationship between groundwater and surface water, between coastal water and fresh water, etc Regulating one system and not the others may not achieve the desired results

Hence, engineering, economic, social, ecological and legal aspects need to be considered,

as well as quantitative and qualitative aspects, and supply and demand Moreover, also the

‘management cycle’ (planning, monitoring, operation & maintenance, etc.) needs to be consistent

Integrated water resources management, then, seeks to manage the water resources in a comprehensive and holistic way It therefore has to consider the water resources from a number of different perspectives or dimensions Once these various dimensions have been considered, appropriate decisions and arrangements can be made

Due to the nature of water, integrated water resources management has to take account of the following four dimensions:

1 the water resources, taking the entire hydrological cycle in account, including stock and flows, as well as water quantity and water quality; distinguishing for instance white, green, grey and blue water

2 the water users, all sectoral interests and stakeholders

3 the spatial scale, including

3.1 the spatial distribution of water resources and uses

3.2 the various spatial scales at which water is being managed, i.e individual user, user groups (e.g user boards), watershed, catchment, (international) basin; and the institutional arrangements that exist at these various scales

4 the temporal scale; taking into account the temporal variation in availability of and demand for water resources, but also the physical structures that have been built to even out fluctuations and to better match the supply with demand

Figure 1.5 Three of the four dimensions of Integrated Water Resources Management

(Savenije, 2000)

Integrated Water Resources Management can now be defined as:

Integrated Water Resources Management (IWRM) is a process which promotes

the coordinated development and management of water, land and related resources, in order to maximise the resultant economic and social welfare in an

equitable manner without compromising the sustainability of vital ecosystems

This is the definition proposed by the Global Water Partnership

Integrated Water Resources Management therefore acknowledges the entire water cycle with all its natural aspects, as well as the interests of the water users in the different sectors

of a society (or an entire region) Decision-making would involve the integration of the different objectives where possible, and a trade-off or priority-setting between these objectives where necessary, by carefully weighing these in an informed and transparent manner, according to societal objectives and constraints Special care should be taken to consider spatial scales, in terms of geographical variation in water availability and the possible upstream-downstream interactions, as well as time scales, such as the natural seasonal, annual and long-term fluctuations in water availability, and the implications of developments now for future generations

To accomplish the integrated management of water resources, appropriate legal, institutional and financial arrangements are required that acknowledge the four dimensions

of IWRM In order for a society to get the right arrangements in place, it requires a sound policy on water

1.4 Policy principles

For a country to change its water management towards a more holistic and integrated management system, it will require to review its water policy This is currently on-going in many countries in Southern Africa, or has been recently concluded A water policy often starts with the definition of a small number of basic principles and objectives, such as the need for sustainable development and desirable socio-economic development

Three key policy principles are known as the three 'E's as defined by Postel (1992):

a) Equity: Water is a basic need No human being can live without a basic volume of fresh water of sufficient quality Humans have a basic human right of access to water resources (see Gleick, 1999) This policy principle is related to the fact that water is often considered a public good Water is such a basic requirement for human life and survival that society has to defend the uses of the water resources in the public interest From here a number of other issues can be derived, such as security (protection against floods, droughts, famine and other hazards)

b) Ecological integrity: Water resources can only persist in a natural environment capable of regenerating (fresh) water of sufficient quality Only sustainable water use can be allowed such that future generations will be able to use it in similar ways as the present generation

c) Efficiency: Water is a scarce resource It should be used efficiently; therefore,

institutional arrangements should be such that cost recovery of the water services should be attained This will ensure sustainability of infrastructure and institutions, but should not jeopardise the equity principle Here comes in the issue of water pricing, and whether or not water should be priced according to its economic value

Much of water resources management deals with finding suitable compromises between these policy principles that sometimes are conflicting

The Southern Africa Vision for Water has been formulated as a desired future characterised by:

Equitable and sustainable utilisation of water for social, environmental justice, regional integration and economic benefit for present and future generations

And the South Africa white paper on water resources has been succinctly summarised as follows:

"Some (water) for all for ever."

Since the appearance of the Brundtland report "Our Common Future" (WCED, 1987), sustainable development has been embraced as the leading philosophy that would on the one hand allow the world to develop its resources and on the other hand preserve unrenewable and finite resources and guarantee adequate living conditions for future generations

Presently the definition most often used of sustainable development is: the ability of the present generation to utilise its natural resources without putting at risk the ability of future generations to do likewise The president of Botswana K Masire stated:

"Our ideals of sustainable development do not seek to curtail development Experience elsewhere has demonstrated that the path to development may simply mean doing more with less (being more efficient) As our population grows, we will certainly have less and less of the resources we have today To manage this situation, we need a new ethic, one that emphasises the need to protect our natural resources in all we do." (cited in Savenije, 2000)

Sustainable development is making efficient use of our natural resources for economic and social development while maintaining the resource base and environmental carrying capacity for coming generations This resource base should be widely interpreted to contain besides natural resources: knowledge, infrastructure, technology, durables and human resources In the process of development natural resources may be converted into other durable products and hence remain part of the overall resource base

Water resources development that is not sustainable is ill-planned In many parts of the world, fresh water resources are scarce and to a large extent finite Although surface water may be considered a renewable resource, it only constitutes 1.5% of all terrestrial fresh water resources; the vast majority is groundwater (98.5%) part of which - at a human scale

- is virtually unreneweable Consequently, there are numerous ways to jeopardise the future use of water either by overexploitation (mining) of resources or by destroying resources for future use (e.g pollution)

Physical sustainability

Physical sustainability means closing the resource cycles and considering the cycles in their integrity (water and nutrient cycles) In agriculture this implies primarily closing or shortening water and nutrient cycles so as to prevent accumulation or depletion of land and water resources: Water depletion results in desertification Water accumulation into water logging Nutrient depletion leads to loss of fertility, loss of water holding capacity, and in general, reduction of carrying capacity Nutrient accumulation results in eutrophication and pollution Loss of top-soil results in erosion, land degradation and sedimentation elsewhere Closing or shortening these cycles means restoring the dynamic equilibria at the appropriate temporal and spatial scales The latter is relevant , since at a global scale all cycles close The question of sustainability has to do with closing the cycles within a human dimension

Economic sustainability

The economic sustainability relates to the efficiency of the system If all societal costs and benefits are properly accounted for, and cycles are closed, then economic sustainability implies a reduction of scale by short-cutting the cycles Efficiency dictates that cycles should be kept as short as possible Examples of short cycles are: water conservation, to make optimum use of rainfall where it falls (and not drain it off and capture it downstream

to pump it up again); water recycling at the spot instead of draining it off to a treatment plant after which it is conveyed or pumped back over considerable distances etc

Strangely enough, economic sustainability is facilitated by an enlargement of scale through trade in land- and water-intensive commodities (the "virtual" water concept) The use of virtual water is an important concept in countries where the carrying capacity of a society

is not sufficient to produce land and water intensive products itself

The closing of cycles should be realised at different spatial scales:

• The rural scale, implying water conservation, nutrient and soil conservation, prevention

of over-drainage and the recycling of nutrients and organic waste

• The urban scale, both in towns and mega-cities, implying the recycling of water, nutrients and waste

• The river basin scale, implying: soil and water conservation in the upper catchment, prevention of runoff and unnecessary drainage and enhancement of infiltration and recharge, flood retention, pollution control and the wise use of wetlands

• The global scale, where water, nutrient and basic resource cycles are integrated and closed The concept of virtual water is a tool for an equitable utilisation of water resources This requires an open and accessible global market and the use of resource-based economic incentives such as resource taxing ("Green tax" which taxes the use of non-renewable or finite resources), as opposed to taxing renewable resources such as labour, which is the general practice today

1.6 Institutional aspects of Integrated Water Resources

Management

The growing complexity of water management induces a need for management at the lowest appropriate level (also known as the ‘subsidiary principle’), resulting in central

government delegating functions to the decentralised organisational (regulatory) and

operational levels In general, the organisational (or regulatory) level may have a mandate over a river basin, while at the operational level concessions may have been delegated to sub-catchment areas or to user groups (municipalities, irrigation districts)

Thus, in managing the resource, a functional differentiation is made between constitutional issues (related to property rights, security, arbitration), organisational issues (regulation, supervision, planning, conflict management), and operational issues (water provision etc.) (World Bank 1993)

These issues will then be handled at three different levels:

• Constitutional level: the activities being governed by conventions of international organisation, bilateral or multilateral treaties and agreements, the national constitution, national legislation or national policy plans

• Organisational level: activities at this level are defined by (federal) state regulation, ministerial regulation, regulation or plan of functional public body (national water authority, (sub) catchment authority), provincial regulation or plan

• Operational level: activities being governed by subcatchment-, district-, town regulations, bye-laws of semi-public or private water users organisations etc

The most important issue in dealing with water resources is to ensure an institutional structure that can coordinate activities in different fields that all have a bearing on water

Linking structures are crucial

Through a process of vertical and horizontal coordination it is possible to integrate different aspects of the water issue at different levels Linking can be facilitated if a country’s water is managed following hydrological boundaries (river basins, which may be subdivided into catchment areas and sub-catchments)

Once agreement exists over what type of functions and decisions can best be made at what level, a next policy option is that of privatisation Operational functions often involve the provision of specific services in water sub-sectors, such as irrigation and drainage, water supply and sanitation, and energy The production function may, in principle, be privatised; but only if the nature of the good (or service) is fit for it, and if government’s

regulatory capacity is strong enough to prevent monopoly formation or other market failures

Financial and economic arrangements are complex issues The maxim ‘water is an economic good and should be priced according to the principle of opportunity costs’, as well as the ‘users pays and polluter pays’ principles carry within them a danger, especially

in countries lacking sufficient resources and with a skewed distribution of wealth In such countries the ‘user pays’ principle may boil down to ‘who can pay is allowed to use or pollute water’ Because of historically grown inequities in society, this may result in a large group of the population having limited access to water resources This often creates severe social problems, and should be considered unconstitutional, as it violates a first order principle (equity)

Therefore a balance has to be found between water pricing which ensures economic sustainability on the one hand, and the social requirement of sufficient access to clean water, on the other (i.e efficiency versus equity)

Instruments that may assist in achieving a balance between efficiency and equity include:

• recovery of real costs by functional (catchment) agencies;

• financial independence (and accountability) of implementing agencies;

• water pricing by means of increasing block tariffs, and other forms of cross-subsidies

A wider concept than water pricing and cost recovery is demand management, which is the

use of economic and legal incentives in combination with awareness raising and education

to achieve more desirable consumption patterns, both in terms of distribution between sectors and quantities consumed, coupled with an increased reliability of supply

In fact, good water management should mean a continuous process of 'integrated demand and supply management', which would seek to match supply with demand through reducing water losses, increasing water yield and decreasing water demand (Savenije and Van der Zaag, 2000)

Environmental sustainability need not conflict with the principle of economic sustainability in a sense that uneconomic activities often waste water resources, if not the resource base itself In addition, environmental costs or ‘environmental externalities’ should be clearly accounted for in economic impact assessments, although this is often not properly done This points to the need for integrating the assessment tools, as suggested by

UNEP (1997): assessments have to be carried out of the likely environmental, economic, and equity impacts of any water resources measure or development, the so-called EIA 3 The vital inclusion of land use appraisal in water management assessment studies is often also omitted Experiences in the field of environmental protection or environmental reconstruction show that positive incentives (e.g subsidies) for practices that restore the ecology are rendering more effect than negative incentives (sanctions, fines) on practices that damage the environment

Another prerequisite for success is the involvement and participation of water users and other stakeholders Control without consensus is hard, if not impossible, to reach The basic premise should be: those who have an interest in the water resource and benefit from

it have the duty to contribute to its management and upkeep (in money and/or in kind) and

have the concomitant right to participate in decision-making This leads to the maxim of

the water boards in The Netherlands: interest - taxation – representation

Moreover, the wider public may play an important role in the difficult process of monitoring this fluid and fugitive resource Formalising the role of interest groups can be realised by applying a comprehensive system of integrated planning at various levels, but

at least at the organisational level

Even a perfect legal and institutional framework (provided that this may ever exist) cannot function without motivated people with sufficient awareness, know-how and skills Human resources are scarce It requires investment in (further) training to build up and maintain the resource

1.7 Strategic issues in water resources management

Current thinking on the crucial strategic issues in water resources is heavily influenced by the so-called Dublin Principles, which were formulated during the International Conference on Water and the Environment in Dublin, 1992, as a preparation for the UN Conference on Environment and Development (UNCED) in Rio de Janeiro the same year During the Rio conference, the concepts of Integrated Water Resources Management were widely discussed and accepted (Table 2.1)

integrated manner

approach, involving all relevant stakeholders

• Women play a central role in the provision, management and safeguarding of water

into account affordability and equity criteria

Associated key concepts:

- An inter-sectoral approach

- Representation of all stakeholders

- Consideration of all physical aspects of the water resources

- Considerations of sustainability and the environment

resource base for future generations

• Decision-making at the lowest possible level (subsidiarity)

Consensus over several issues have emerged in the last few years:

- In terms of water allocation, basic human needs have priority; other uses should be prioritised according to societal needs and socio-economic criteria

- The river basin is the logical unit for water resources management

- Participatory approaches in decision-making, and the crucial role of women

There are a number of important outstanding issues of debate:

- Privatisation, and more generally the role of the private sector in water management

- The value of water (the social, economic and ecological value)

- The pricing of water (whether we should price basic needs, and if so, how we can safeguard access to water by the poor)

- Water for food (potential conflict between irrigation and ecological water demands and the scope for improving rainfed-agriculture)

- Non-water borne sanitation or traditional water borne end-of-pipe sanitation

It is obvious that these remaining issues are very important strategically Countries are currently dealing with them individually It is sometimes feared that outside pressure may

in cases lead to countries making the wrong decision, and by so doing jeopardising fundamental policy principles This may, for instance, be the case when a water utility is privatised without the country having an effective regulatory body to supervise the operations of the privatised utility

1.8 Exercises

1a What are in your opinion the main policy issues for the water sector in your country? 1b Which objectives for the management of water resources can be derived from that? 1c What would be suitable performance criteria for these objectives?

1d Which institutions should be responsible for the implementation of these objectives? 1e Which should the tasks and responsibilitIes be for these institutions?

2 Sketch the debate between professionals who promote water borne sanitation versus the ones that promote non-water borne sanitation

3 Sketch the debate between those professionals and stakeholders that promote

privatisation versus the ones that are against it

1.9 References

Falkenmark, Malin, 1995, Coping with water scarcity under rapid population growth Paper presented at the Conference of SADC Water Ministers Pretoria, 23-24 November 1995

Gleick, P., 1999, The Human Right to Water Water Policy 1(5): 487-503

ICWE, 1992, The Dublin Statement and Report of the Conference International conference on water and the environment: development issues for the 21st century; 26-31 January 1992, Dublin

Pallett, J., 1997, Sharing water in Southern Africa Desert Research Foundation of Namibia,

Windhoek

Postel, Sandra, 1992, Last oasis, facing water scarcity W.W Norton, New York

Savenije, H.H.G., 2000, Water resources management: concepts and tools Lecture note IHE, Delft and University of Zimbabwe, Harare

Savenije, H.H.G., and P van der Zaag, 2000, Conceptual framework for the management of shared

river basins with special reference to the SADC and EU Water Policy 2 (1-2): 9-45

UNEP, 1997, The fair share water strategy for sustainable development in Africa UNEP, Nairobi

WCED, 1987, Our common future Report of the Brundtland Commission Oxford University

Press, Oxford

World Bank, 1993, Water resources management; a World Bank Policy Paper World Bank, Washington DC

2 Water resources (Savenije, 2000)

The origin of water resources is rainfall As rainfall reaches the surface it meets the first

separation point At this point part of the rainwater returns directly to the atmosphere,

which is called evaporation from interception I The remaining rainwater infiltrates into the

soil until it reaches the capacity of infiltration This is called infiltration F If there is

enough rainfall to exceed the interception and the infiltration, then overland flow (also

called surface runoff) Q s is generated The overland flow is a fast runoff process, which

generally carries soil particles A river that carries a considerable portion of overland flow

has a brown muddy colour and carries debris

The infiltration reaches the soil moisture Here lies the second separation point From the

soil moisture part of the water returns to the atmosphere through transpiration T If the soil

moisture content is above field capacity (or if there are preferential pathways) part of the

soil moisture percolates towards the groundwater The reverse process of percolation is

capillary rise The percolation feeds the groundwater and renews the groundwater On

average the percolation minus the capillary rise equals the seepage of groundwater Q g to

the surface water The seepage water is clean and does not carry soil particles A river that

has clear water carries water that stems from groundwater seepage This is the slow

component of runoff During the rise of a flood in a river when the water colour is brown,

the water stems primarily from overland flow During the recession of the flood, when the

water is clear, the river flow stems completely from groundwater seepage

The water that is consumed by the vegetation through transpiration is called "green water"

It is an important water resource for agriculture, nature and livestock The surface water

and groundwater which are intimately intertwined are the "blue water" Although the

groundwater and surface water cannot be separated and although surface water consists to

a large extent of groundwater, they are often dealt with separately This is because they

have quite different characteristics (time scales, quantities, availability) and because they

obey different laws of motion

2.1 The water balance

In the field of hydrology the budget idea is widely used Water balances are based on the

principle of continuity This can be expressed with the equation:

t

S

= O(t) -

I(t)

∆

∆

(2.1)

where I is the inflow in [L3/T], O is the outflow in [L3/T], and ∆S/∆t is the rate of change

in storage over a finite time step in [L3/T] of the considered control volume in the system

The equation holds for a specific period of time and may be applied to any given system

provided that the boundaries are well defined Other names for the water balance equation

are Storage Equation, Continuity Equation and Law of Conservation of Mass

Several types of water balances can be distinguished, including:

• the water balance of the earth surface;

• the water balance of a drainage basin;

• the water balance of the world oceans;

• the water balance of the water diversion cycle (human interference);

• the water balance of a local area like a city, a forest, or a polder

The water balance of the earth is given in tables 2.1 and 2.2 The water balance of some

rivers is given in table 2.3

Amount of water Water occurrence 1012 m3

% of all water % of fresh water

1012 m2 1012 m3/a 1012 m3/a 1012 m3/a 1012 m3/a

Table 2.3 Indicative average annual water balances for the drainage basins of some

of the great rivers

Water balance of a drainage basin

The water balance is often applied to a river basin A river basin (also called watershed,

catchment, or drainage basin) is the area contributing to the discharge at a particular river

cross-section The size of the catchment increases if the point selected as outlet moves

downstream If no water moves across the catchment boundary indicated by the broken

line, the input equals the precipitation P while the output comprises the evapotranspiration

E and the river discharge Q at the outlet of the catchment Hence, the water balance may be

written as:

t

S

= Q - E

In this formula, care should be taken to use the same units for all parameters, e.g

mm/month or m3/month

∆S, the change in the amount of water stored in the catchment, is difficult to measure

However, if the ‘account period’ for which the water balance is established is taken

sufficiently long, the effect of the storage term becomes less important, as precipitation and

evapotranspiration accumulate while storage varies within a certain range When

computing the storage equation for annual periods, the beginning of the balance period is

preferably chosen at a time that the amount of water in store is expected not to vary much

for each successive year These annual periods, which do not necessarily coincide with the

calendar years, are known as hydrologic - or water years The storage equation is

especially useful to study the effect of a change in the hydrologic cycle

If ∆S/∆t may be neglected, equations 2.1 and 2.2 may be re-written as:

O(t)

and

Q E

If the evaporation term E consists of Interception I and Transpiration T, then

T I

and

Q T I

How to determine the blue and green water on an annual basis?

Precipitation (P) and the blue water (Q) can be determined through measurement The

difficulty lies with the green water (T) We first concentrate on the interception term (I)

The white water (I) consists of the direct evaporation from small stagnant pools, bare soil

evaporation and interception Savenije (1997) showed that under the assumption that the

soil moisture storage variation at a monthly or annual time step is small, the value for

interception can be computed as:

D)(P,Min

=

where: D is the threshold evaporation (from interception) on a monthly or annual basis

The effective precipitation can now be defined as the remainder of the rainfall after

interception has occurred:

) D, P (

=

After interception has occurred, water will either become blue water (through groundwater

or surface flow), or become green water

From gauged data of Q and P, and given the threshold value D, the effective runoff

coefficient c, on a water year basis, can be calculated as follows:

D P

Q P

Q

=

c

where P and Q are the annual rainfall and runoff on a water year basis

The runoff coefficient indicates the part of the effective precipitation that will become blue

water Thus, blue water can now be defined as:

) D, P ( c

= P c

Transpiration must now be the balance between the effective precipitation and blue water:

) D, P ( ) c (

= P ) c (

Equations 2.7, 2.10 and 2.11 complete the "rainbow of water" (see Figure 1.3) Equation

2.7 accounts for the white water; eq 2.11 for the green water, and eq 2.10 for the blue

water

To find adequate values for I and T now depends on finding an appropriate value for D

Figure 2.1 presents the distribution of monthly values of rainfall P, direct evaporation from

interception I, transpiration T, and runoff R the total evaporation E over time in the

Pungwe catchment in Mozambique Of the total rainfall, only the evaporation from

interception is a loss to the water resources in the catchment The remainder is the green

water and the blue water

catchment

2.2 Groundwater resources

Groundwater can be split up into fossil groundwater and renewable groundwater Fossil groundwater should be considered a finite mineral resource, which can be used only once, after which it is finished Renewable groundwater is groundwater that takes an active part

in the hydrological cycle The latter means that the residence time of the water in the surface has an order of magnitude relevant to human planning and considerations of sustainability The limit between fossil and renewable groundwater is clearly open to debate Geologists, that are used to working with time scales of millions of years would only consider groundwater as fossil if it has a residence time over a million years A hydrologist might use a time scale close to that However, a water resources planner should use a time scale much closer to the human dimension, and to the residence time of pollutants

sub-Figure 2.2 Blue water is surface runoff plus seepage from renewable groundwater

Fossil GW Renewable GW Overland flow

Slow runoff Fast runoff

Blue water

In our definition, the renewable groundwater takes active part in the hydrological cycle and hence is "blue water" Groundwater feeds surface water and vice versa In the Mupfure catchment in Zimbabwe, Mare (1998) showed that more than 60% of the total runoff of the catchment originated from groundwater Hence most of the water measured at the outfall was groundwater One can say that all renewable groundwater becomes surface water and

most of the surface water was groundwater

Two zones can be distinguished in which water occurs in the ground:

• the saturated zone

• the unsaturated zone

For the hydrologist both zones are important links and storage devices in the hydrological

cycle: the unsaturated zone stores the "green water", whereas the saturated zone stores the

"blue" groundwater For the engineer the importance of each zone depends on the field of

interest An agricultural engineer is principally interested in the unsaturated zone, where

the necessary combination of soil, air and water occurs for a plant to live The water

resources engineer is mainly interested in the groundwater which occurs and flows in the

saturated zone

The type of openings (voids or pores) in which groundwater occurs is an important

property of the subsurface formation Three types are generally distinguished:

1 Pores: openings between individual particles as in sand and gravel Pores are

generally interconnected and allow capillary flow for which Darcy’s law (see below)

can be applied

2 Fractures, crevices or joints in hard rock which have developed from breaking of the

rock The pores may vary from super capillary size to capillary size Only for the

latter situation application of Darcy’s law is possible Water in these fractures is

known as fissure or fault water

3 Solution channels and caverns in limestone (karst water), and openings resulting

from gas bubbles in lava These large openings result in a turbulent flow of

groundwater which cannot be described with Darcy’s law

The porosity n of the subsurface formation is that part of its volume which consists of

openings and pores:

V

V

where: V p is the pore volume and V is the total volume of the soil

When water is drained by gravity from saturated material, only a part of the total volume is

released This portion is known as specific yield The water not drained is called specific

retention and the sum of specific yield and specific retention is equal to the porosity In

fine-grained material the forces that retain water against the force of gravity are high due

to the small pore size Hence, the specific retention of fine-grained material (silt or clay) is

larger than of coarse material (sand or gravel)

Groundwater is the water which occurs in the saturated zone The study of the occurrence

and movement of groundwater is called groundwater hydrology or geohydrology The

hydraulic properties of a water-bearing formation are not only determined by the porosity

but also by the interconnection of the pores and the pore size

An aquifer is a water-bearing layer for which the porosity and pore size are sufficiently

large to allow transport of water in appreciable quantities (e.g sand deposits)

Groundwater flow

The theory on groundwater movement originates from a study by the Frenchman Darcy,

first published in 1856 From many experiments he concluded that the groundwater

discharge Q is proportional to the difference in hydraulic head ∆H and cross-sectional area

A and inversely proportional to the length ∆s, thus

s

H

* k

* A -

= v

* A

where k, the proportionality constant, is called the hydraulic conductivity, expressed in

m/d; and v is the specific discharge, also called the filter velocity Since the hydraulic head

decreases in the direction of flow, the filter velocity has a negative sign

Groundwater as a storage medium

For the water resources engineer groundwater is a very important water resource for the

following reasons:

• it is a reliable resource, especially in climates with a pronounced dry season

• it is a bacteriologically safe resource, provided pollution is controlled

• it is often available in situ (wide-spread occurrence)

• it may supply water at a time that surface water resources are limited

• it is not affected by evaporation loss, if deep enough

It also has a number of disadvantages:

• it is a limited resource, extractable quantities are often low as compared to surface water

resources

• groundwater recovery is generally expensive as a result of pumping costs

• groundwater, if phreatic, is very sensitive to pollution

• groundwater recovery may have serious impact on land subsidence or salinisation

• groundwater is often difficult to manage

Especially in dry climates the existence of underground storage of water is of extreme

importance The water stored in the subsoil becomes available in two ways One way is by

artificial withdrawal (pumping), the other is by natural seepage to the surface water

The latter is an important link in the hydrological cycle Whereas in the wet season the

runoff is dominated by surface runoff, in the dry season the runoff is almost entirely fed by

seepage from groundwater (base flow) Thus the groundwater component acts as a

reservoir which retards the runoff from the wet season rainfall and smoothens out the

shape of the hydrograph

The way this outflow behaves is generally described as a linear reservoir, where outflow is

considered proportional to the amount of storage:

S K

=

where K is a conveyance factor of the dimension s-1 Eq 2.14 is an empirical formula

which has some similarity with the Darcy equation (Eq 2.13) In combination with the

water balance equation, and ignoring the effect of rainfall P and evaporation E, Eq 2.14

yields an exponential relation between the discharge Q and time t

t

* K -

t S(

=

and hence, using Eq.(2.14):

) t t K

t Q(

=

(2.16)

Eq (2.16) is useful for the evaluation of surface water resources in the dry season

Fig 2.3 gives a typical hydrograph, indicating flow from surface runoff and groundwater

The depletion curve has the shape of a negative exponential function, in keeping with the

Darcy equation Compare with the hydrograph of the Pungwe river (Fig 2.4)

Figure 2.3: A typical hydrograph

2.3 Surface water

Surface water resources are water resources that are visible to the eye They are mainly the

result of overland runoff of rain water, but surface water resources can also originate from

groundwater, as was stated in Section 4.1 As Mare (1998) pointed out, more than 60% of

the surface water in the Mupfure basin stemmed from groundwater, a resource hitherto

disregarded Surface water is linked to groundwater resources through the processes of

infiltration (from surface water to groundwater) and seepage (from groundwater to surface

water) Surface water occurs in two kinds of water bodies:

• water courses, such as rivers, canals, estuaries and streams;

• stagnant water bodies, such as lakes, reservoirs, pools, tanks, etc

The first group of water bodies consists of conveyance links, whereas the second group

consists of storage media Together they add up to a surface water system

The amount of water available in storage media is rather straightforward as long as a

relation between pond level and storage is known The surface water available in channels

is more difficult to determine since the water flows The water resources of a channel are

defined as the total amount of water that passes through the channel over a given period of

time (e.g a year, a season, a month) In a given cross-section of a channel the total

available amount of surface water runoff over a time step ∆t is defined as the average over

time of the discharge

∫

+

=

t t

t

dt Q

1 R

∆

The discharge Q is generally determined on the basis of water level recordings in

combination with a stage discharge relation curve, called a rating curve A unique

relationship between water level and river discharge is usually obtained in a stretch of the

river where the river bed is stable and the flow is slow and uniform, i.e the velocity pattern

does not change in the direction of flow Another suitable place is at a calm pool, just

upstream of a rapid Such a situation may also be created artificially in a stretch of the river

(e.g with non-uniform flow) by building a control structure (threshold) across the river

bed The rating curve established at the gauging station has to be updated regularly,

because scour and sedimentation of the river bed and river banks may change the stage

discharge relation, particularly after a flood

The rating curve can often be represented adequately by an equation of the form:

H - H a

=

where Q is the discharge in m3/s, H is the water level in the river in m, H 0 is the water level

at zero flow, and a and b are constants The value of H 0 is determined by trial and error

The values of a and b are found by a least square fit using the measured data, or by a plot

on logarithmic paper and the fit of a straight line (see Fig 2.2)

Figure 2.5 Rating curve in Limpopo river at Sicacate

Fig 2.5 shows the rating curve of the Limpopo river at Sicacate; the value of b equals 1.90 The Limpopo is an intermittent river which falls dry in the dry season and can have very

high flash floods during the flood season The station of Sicacate has a value of H 0 equal to 2.1 m In Fig 2.4 a clear flood branch can be distinguished which is based on peak flows recorded during the floods of 1981, 1977 and 1978 in the Limpopo river The gradient of a flood branch becomes flat as the river enters the flood plain; a small increase in water level then results in a large increase in discharge

To illustrate the trial and error procedure in determining the value of H 0, a plot of data with

H 0 =0 has been added It can be seen that the value of H 0 particularly affects the determination of low flow

2.4 Catchment yield

Water resources engineers are primarily concerned with catchment yields and usually study hydrometric records on a monthly basis For that purpose short duration rainfall should be aggregated In most countries monthly rainfall values are readily available To determine catchment runoff characteristics, a comparison should be made between rainfall and runoff For that purpose, the monthly mean discharges are converted first to volumes

per month and then to an equivalent depth per month Q over the catchment area Rainfall P and runoff Q being in the same units (e.g in mm/month) may then be compared

A typical monthly rainfall pattern is shown in Fig 2.6 for the catchment of the Cunapo river in Trinidad The monthly runoff has been plotted on the same graph Fig 2.7 shows

the difference between Q and P, which partly consists of evaporation E (including

interception, open water evaporation, bare soil evaporation and transpiration) and partly is caused by storage On a monthly basis one can write:

t / S - E - P

=

The presence of the evaporation and the storage term makes it difficult to establish a

straightforward relation between Q and P The problem is further complicated in those

regions of the world that have distinctive rainy and dry seasons In those regions the

different situation of storage and evaporation in the wet and dry season make it difficult to

establish a direct relation

Figure 2.6 Monthly mean rainfall and runoff in the Cunapo catchment

Figure 2.7 Mean monthly losses and change in storage in the Cunapo catchment

Figure 2.8 Rainfall plotted versus runoff in the Cunapo river basin

Fig 2.8 shows the plot of monthly rainfall P against monthly runoff Q for a period of four

years in the Cunapo catchment in Trinidad The plots are indicated by a number which signifies the number of the month The following conclusions can be drawn from studying the graph

• There appears to be a clear threshold rainfall below which no runoff takes place This threshold value is the result of evaporation from intercepted rainfall (interception) It is the direct evaporation from wet leaves, the wet surface and the upper layer of the soil

• It can be seen that the same amount of rainfall gives considerably more runoff at the end of the rainy season than at the start of the rainy season The months with the numbers 10, 11 and 12 are at the end of the rainy season, whereas the rainy season begins (depending on the year) in the months of May to July At the start of the rainy season the contribution of seepage to runoff is minimal, the groundwater storage is virtually empty and the amount to

be replenished is considerable; the value of ∆S/∆t in Eq.(2.19) is thus positive, reducing the

runoff R At the end of the rainy season the reverse occurs

After the interception I has been subtracted from the rainfall the remainder: the effective rainfall can be thought to be split up between superficial runoff Q s and infiltration F The infiltration replenishes the soil moisture, which feeds the transpiration T If the water

holding capacity of the soil is exceeded, the remainder of infiltration recharges the

groundwater This recharge R joins the groundwater storage which through seepage Q g

contributes to runoff

The sum of Q s and Q g is the total runoff Q of Eq (2.19) The total evaporation E consists

of the sum of I and T At a monthly time scale, the storage S is the sum of the water stored

in the groundwater, in the soil and in reservoirs Only in very large catchments (e.g the Zambezi) is there a measurable storage in the watercourses By taking into account a

threshold D for interception and the storage S, a relation can be obtained between Q and P

2.5 The rainbow of water revisited

Of all water resources, "green water" is probably the most under-valued resource Yet it is responsible for by far the largest part of the world's food and biomass production The concept of "green water" was first introduced by Falkenmark (1995), to distinguish it from

"blue water", which is the water that occurs in rivers, lakes and aquifers The storage medium for green water is the unsaturated soil The process through which green water is consumed is transpiration Hence the total amount of green water resources available over

a given period of time equals the accumulated amount of transpiration over that period In this definition irrigation is not taken into account Green water is transpiration resulting directly from rainfall, hence we are talking about rainfed agriculture, pasture, forestry, etc The average residence time of green water in the unsaturated zone is the ratio of the storage to the flux (the transpiration) At a global scale the soil moisture availability is 440

mm (see Tables 2.1 and 2.2: 65/149) In tropical areas the transpiration can amount to 100 mm/month Hence the residence time of green water in tropical areas is approximately 4 months This residence time, however, applies to deeply rooting vegetation For shallow rooting vegetation the residence time in the root zone is much shorter In temperate and polar areas where transpiration is significantly less the residence is much longer At a local scales, depending on climate, soils and topography, these numbers can vary significantly

Green water is a very important resource for global food production About 60% of the world staple food production relies on rainfed irrigation, and hence green water The entire meat production from grazing relies on green water, and so does the production of wood from forestry In Sub-Saharan Africa almost the entire food production depends on green water (the relative importance of irrigation is minor) and most of the industrial products, such as cotton, tobacco, wood, etc

There is no green water without blue water, as their processes of origin are closely related Blue water is the sum of the water that recharges the groundwater and the water that runs-off over the surface Blue water occurs as renewable groundwater in aquifers and as surface water in water bodies These two resources can not simply be added, since the recharge of the renewable groundwater eventually ends up in the surface water system Adding them up often implies double counting Depending on the climate, topography and geology, the ratio of groundwater recharge to total blue water varies In some parts the contribution of the groundwater to the blue water can be as high as 70-80%, in some parts (on solid rock surface), it can be negligible Generally the groundwater contribution to the blue water is larger than one thinks intuitively The reason that rivers run dry is more often related to groundwater withdrawals, than to surface water consumption

Engineers always have had a preference for blue water For food production, engineers have concentrated on irrigation and neglected rainfed agriculture, which does not require impressive engineering works Irrigation is a way of turning blue water into green water Drainage is a way of turning green water into blue water

To complete the full picture of the water resources, besides green water and blue water, there is "white water" White water is the part of the rainfall that feeds back directly to the atmosphere through evaporation from interception and bare soil Some people consider the white water as part of the green water, but that adds to confusion since green water is a productive use of water whereas the white water is non-productive The white and green

water together form the vertical component of the water cycle, as opposed to the blue water, which is horizontal In addition, the term white water can be used to describe the rainfall which is intercepted for human use, including rainwater harvesting

Table 2.4 Global Water Resources, fluxes, storage and average residence times

Resource Fluxes [L/T] or [L3/T] Life

note: transpiration and interception fluxes apply to tropical areas

storage in the root zone can be significantly less than 440 mm

*) indicate rough estimates

Table 2.4 presents the quantities of fluxes and stocks of these water resources, and the

resulting average residence times, at a global scale The stocks S u , S s , S w , S g , S a and S o

represent the life storages of the unsaturated zone, the surface, the water bodies, the renewable groundwater, the atmosphere and the oceans, respectively For catchments and sub-systems similar computations can be made The relative size of the fluxes and stocks can vary considerably between catchments Not much information on these resources exists at sub-catchment scale

The study of the Mupfure catchment in Zimbabwe by Mare (1998) is an exception Table 4.6 illustrates the importance of green water and renewable groundwater in a country where these resources have been mostly disregarded Fig 2.9, based on 20 years of records (1969-1989) in the Mupfure basin in Zimbabwe (1.2 Gm2), shows the separation of rainfall into interception (White), Green and Blue water The model used for this separation is described by Savenije (1997) (see equations 2.7, 2.10 and 2.11 above) It can be seen that there is considerably more green water than blue water available in the catchment Moreover, the model showed that more than 60% of the blue water resulted from groundwater

Partitioning of Rainfall

Mupfure river basin

0 500 1000 1500

Rainfall (mm/a)

G G+B W+G+B

Partitioning of Rainfall

Mupfure river basin

Figure 2.9 Partitioning of rainfall between "White", "Green" and "Blue" water in the

Mupfure sub-catchment in Zimbabwe (records of 1969-1989)

Table 2.5 Water resources partitioning and variability in the Mupfure River Basin,

Resource type Rainfall (P) "White" (W) "Green" (G) "Blue" (B)

It can be seen from Table 2.5 that the variability of the "white" water is much lower (11%)

than the variability of the "green" (67%) and "blue" water (69%) This is a general

phenomenon which can be understood from the fact that interception is the first process to

occur and that this process has an upper boundary The maximum amount of interception

per day is limited by the amount of interception storage and the potential evaporation

2.6 The water balance as a result of human interference

Attempts have been made to incorporate the interference of man in the hydrological cycle

through the introduction of the water diversion cycle, which includes water withdrawal and

water drainage This diversion cycle is exerting significant influence on the terrestrial

water cycle, especially in highly economically developed regions with a dense population

(See Fig 2.10)

The water diversion cycle including human interference results in the following annual

average water balance equation (neglecting storage variation):

Q + C + E

= D

+

D + H + R - R - U + U

=

where: P = precipitation

E =T+I+O=total evaporation from the land surface (transpiration + interception +

open water evaporation)

C = net water consumption due to water use

Q = runoff from land to ocean

U s +U g = intake from surface and groundwater

R s +R g = return flows to surface and groundwater

H = rainwater harvesting

D = desalination

Matalas, 1987)

2.7 References

Darcy, H.P.G., 1856 Les fontaine publiques de la ville de Dijon, V Dalmont, Paris, 647 p

Falkenmark, Malin, 1995, Coping with water scarcity under rapid population growth Paper presented at the Conference of SADC Water Ministers Pretoria, 23-24 November 1995 Mare, A., 1998 Green water and Blue water in Zimbabwe: the Mupfure river basin case MSc thesis, DEW.044, IHE, Delft, The Netherlands

Rodda, J.C and N.C Matalas, 1987 Water for the future; Hydrology in perspective Proceedings

of the Rome Symposium IAHS publ no 164

Savenije, H.H.G., 1997 Determination of evaporation from a catchment water balance at a monthly time scale, Hydrology and Earth System Sciences, no.1, 1997

Savenije, H.H.G., 2000, Water resources management: concepts and tools Lecture note IHE, Delft and University of Zimbabwe, Harare

3 Water allocation principles

3.1 Introduction

An important purpose of water management is to match or balance the demand for water with its availability, through suitable water allocation arrangements Water availability is dealt with in other courses (e.g Hydrology) This lecture note of Water Using Activities aims to provide tools to estimate the demand for water for different types of use

There are a large number of types of water use Among these are:

Domestic use in urban centres

Domestic use in rural areas

Livestock

Industrial use

Commercial use

Institutions (e.g schools, hospitals etc.)

Cooling (e.g for thermal power generation)

Waste and wastewater disposal

Navigation

Recreation

Fisheries

The environment (wildlife, nature conservation etc.)

Demand for water is the amount of water required at a certain point The use of water

refers to the actual amount reached at that point

We can distinguish withdrawal uses and non-withdrawal (such as navigation, recreation, waste water disposal by dilution) uses; as well as consumptive and non-consumptive uses

Consumptive use is the portion of the water withdrawn that is no longer available for further use because of evaporation, transpiration, incorporation in manufactured products and crops, use by human beings and livestock, or pollution

The terms “consumption”, “use” and “demand” are often confused The amount of water actually reaching the point where it is required will often differ from the amount required Only a portion of the water used is actually consumed, i.e lost from the water resource system

3.2 Balancing demand and supply

There are various ways how to allocate water The challenge is to find an optimal

allocation that, firstly, adheres to laid-down legal and other regulations, and secondly, satisfies the water demand of all users as much as possible Or,